Device for the generation of a carrier for an interferogramme

ABSTRACT

A process for generating a carrier in an interferogram in an interferometry device either of the “real-time holographic interferometry” type or of the “double-exposure holographic interferometry” type, including recording a first wave front on a hologram and coming from an object; generating a second wave front, which two wave fronts are superposed; orthogonally polarizing the superposed wave fronts, either naturally or by a modification stage; and passing the orthogonally polarized two wave fronts through a birefringent crystal, a polarizer and a detector to make these two wave fronts interfere.

The present invention relates to the area of interferograms, that is, tothe recording of an interference figure produced by the superpositioningof two waves. It concerns in particular but not exclusively the creationor the obtention of a carrier in interferogram systems in which acarrier is introduced for studying and analyzing any shifts of theobject observed.

In a more precise manner, the present invention relates to theintroduction of a carrier in interferograms of the type “real-timeholographic interferometry” or “double-exposure holographicinterferometry”.

Interferograms are used to measure, among other things, minutedeformations in structures.

Holography, a technique used to obtain interferograms, is a method ofrecording by interference between two beams of coherent light. The firstof these beams issues from the light diffused or transmitted by theobject. The second beam serves as reference. The two beams interfere onthe holographic support, where they record a hologram. The subsequentreadback [rereading, playback] of this hologram by the reference beamreconstructs the wave front of the first beam as it was during therecording.

In holographic interferometry, well-known to the expert in the art,called “in real time”, two wave fronts are compared at instant t1:

-   -   A wave front recorded previously on the hologram at instant t0        and restored by readback of the hologram by a reference beam.        This wave front corresponds to the image of the object as it was        at instant t0.    -   The wave front coming from the object and transmitted by the        hologram.

These two wave fronts interfere on the detector, e.g., a camera focusedon the object, thus producing an interferogram.

In holographic interferometry, well-known to the expert in the art,called “double exposure”, two wave fronts are compared at instant t2:

-   -   A wave front previously recorded on the hologram at instant t0        and restored by readback of the hologram by a first reference        beam. This wave front corresponds to the image of the object as        it was at instant t0.    -   A second wave front previously recorded on the hologram at        instant t1 and restored by readback of the hologram by a second        reference beam. This wave front corresponds to the image of the        object as it was at instant t1.

These two wave fronts interfere on the detector, e.g., a camera focusedon the object, thus producing an interferogram.

Whether it is in “real-time” or “double-exposure” interferometry, ananalysis of the interferogram allows a going back [return] to the shiftsundergone by the surface of the object between the two instants t0 andt1 of the measuring.

There are two techniques for analyzing interferograms that allow a goingback to the phase difference between the two wave fronts. The firsttechnique concerns the techniques of phase shift that necessitate theacquisition of several interferograms. The second technique requiresonly one single interferogram for going back to the variation in phase.In this instance, for the analysis with a single interferogram a carrierfrequency or simply called (carrier) must be introduced onto theinterferogram.

Therefore, in order to calculate the variation in phase between the twowave fronts that served to acquire a single interferogram, it isnecessary to introduce a “carrier” onto the interferogram. This carrieris obtained by introducing an air wedge between the two wave fronts,that is, by slightly inclining one wave front relative to the other one.This inclination causes a linear variation of the phase along an axis ofthe image, e.g., x. This has the effect of introducing fringes into theinterferogram.

If the object did not move between t0 and t1 the fringes are parallel,perpendicular to the x axis, and equidistant along x. On the other hand,if the object moved between the two exposures the deformations of thesefringes teach us about the deformations undergone by the objectobserved. An analysis of these deformations can be made in differentmanners: by Fourier transformation, interpolation of fringes, etc.

The systems used to create this carrier are, e.g., of an electro-opticaltype or mechanical type by shifting a mirror on the reference beam inthe case of interferometry “in real time” or one of the two referencebeams in the case of “double-exposure” interferometry. These systemsconsequently require a control means. These systems are thereforeexpensive and also have the disadvantage of not being able to beenvisaged when the holographic material studied is too thick.

The present invention has the purpose of remedying the disadvantages ofthe prior art by proposing a system in which no shifting is necessaryany more for generating the carrier.

It proposes to this end a process for the generation of a carrier in aninterferogram in an interferometry device.

For interferometry “in real time” the process comprises a first stage ofrecording on a hologram of a wave front coming from an object. During asecond stage this wave front is restored by readback of the hologram bya reference beam. At the same time, this wave front is superposed on anew wave front coming from the object observed.

For the “double-exposure” interferometry the process comprises a firststage of recording on a hologram of a wave front coming from an object.It comprises a second stage of recording on the hologram of a new wavefront coming from the object. These two wave fronts are restored byreadback of the hologram by two reference beams and are superposed.Thus, the second wave front is obtained in the case of “double-exposure”interferometry by the recording of a new wave front coming from theobject.

The process is characterized in that the two above-cited wave fronts arepolarized orthogonally, either naturally or by a modification stage, andin that it comprises a stage of the passage of these two wave frontspolarized orthogonally through [by] a birefringent crystal, a polarizerand a detector in order to make these two wave fronts interfere.

According to a first embodiment of the invention the modification stageconsists of the modification of the wave coming from the object or ofthe wave of the reference beam in the case of interferometry “in realtime”, or of one of the two reference beams in the case of“double-exposure” interferometry by means of an electro-optical switch.This modification of the polarization of one of these waves takes placeafter the recording of the hologram.

According to a second embodiment of the invention that relates tointerferometry “in real time” in the case in which the polarization isobtained naturally, the holographic material used in the interferometrydevice is an anisotropic material that authorizes the processes ofanisotropic diffraction, namely, that the polarization of the wave frontreconstructed by diffraction of the reference beam on the hologram isorthogonal to the polarization of the reference beam and to that of thebeam coming from the object.

The present invention also relates to the device for carrying out theprocess for the generation of a carrier in an interferogram of the“real-time holographic interferometry” type or of the “double-exposureholographic interferometry” type, comprising a first stage of recordingon a hologram, of a first wave front coming from an object 8, thegeneration of a second wave front, in which the two wave fronts,polarized orthogonally, are superposed, comprising a plurality ofobjectives 3, 6, 7 or lenses, at least one polarizer 5, at least oneholographic environment 1, and at least one detector 9, characterized inthat it comprises a birefringent crystal and that the two wave frontspass through this crystal and the polarizer in order to make these twowave fronts interfere on the detector.

According to a possibility offered by the invention, in order to realizethe stage of modifying the wave coming from the object or the referencebeam, the device comprises an electro-optical switch.

According to an embodiment of the invention the birefringent crystal isin the form of a birefringent biprism situated in the vicinity of animage plane from the optical viewpoint of the object observed.

According to a second embodiment of the invention the birefringentcrystal is in the form of a birefringent lamina [strip, blade]. In thisinstance the birefringent lamina is ideally situated at infinity from anoptical viewpoint relative to the object observed.

The invention will be better understood with the aid of the followingdescription given purely by way of explanation, and in the case of“real-time” interferometry, of an embodiment of the invention withreference made to the attached figures.

FIG. 1 shows a schematic view of a first embodiment of the device inaccordance with the invention.

FIG. 2 shows a schematic view of a second embodiment of the device inaccordance with the invention.

FIG. 3 shows a part of FIG. 2 in a more precise manner.

FIG. 4 shows a schematic section of the birefringent lamina used in thesecond embodiment.

A first embodiment of the device in accordance with the invention isshown in FIG. 1, whereas the second embodiment is shown in and explainedwith FIGS. 2 to 4. In both instances the introduction of the air wedgebetween the two wave fronts is obtained by using a birefringent crystal.In these two instances, in order to interfere the two wave frontspolarized orthogonally, these two wave fronts pass through a commonpolarizer placed between the birefringent crystal and the detector.

In the first embodiment the switch or electro-optical componentpermitting the polarization of one of the wave fronts to be rotated wasnot shown. Thus, it is supposed that the material of holographicenvironment 1 used is anisotropic. These anisotropic materials, whenthey are employed in a configuration of anisotropic diffraction, thatwill not be described any more in the following because it is well-knownto an expert in the art, have the consequence that the diffracted beamis polarized orthogonally to the polarization of the beam object. Inthis instance the two wave fronts employed to create the interferogramare polarized orthogonally without requiring the use of anyelectro-optical switch.

In the embodiment of FIG. 1, polarizer 2 has the function of ensuringthat the beam incident on holographic environment 1 is well-polarized.The objective, or lens permits the image of the object to be formed inthe vicinity of the center of birefringent biprism 4. This biprism 4 is,e.g., a biprism of “Wallaston”, of “Rochon” or of “Nomarski”, well-knownto the expert in the art.

The orthogonal polarization of the two wave fronts are selected in sucha manner that they correspond to the two neutral lines of biprism 4.This has the effect of introducing an angle between the wave fronts. Thepolarizations of the two wave fronts are then projected on the samepolarizer 5. The optical system formed by the two lenses 6, 7 permitsthe image of object 8 to be re-formed on detector 9, in the presentinstance the detection camera on which the interferogram is formed withthe fringes of the carrier. The increment size [space; pitch] of thefringes of this carrier is defined by the characteristics of biprism 4.

In order that the fringes are localized on object 8, the separationplane of the bundles should be situated in the vicinity of an image,which means that biprism 4 should be located in the vicinity of an imageof the object observed.

In the second embodiment the various elements present in the firstembodiment are also present, but instead of using a biprism 4 placed inthe vicinity of an image plane of object 8 a birefringent lamella 10 isused that is placed or situated this time at infinity (according to theoptical understanding of the term “infinity”) relative to the object andthe infinity is brought to the focus of lens 6 in FIG. 2.

As is illustrated in FIG. 3, each element (pixel) of the imagesassociated with a light cone 11. In the instance in which the opticalpupil is situated at infinity, the axis of this cone is the same for allthe pixels of the image and is parallel to the z axis of the assembly[mounting, layout].

At infinity, that is, in the plane of birefringent lamina 10, each pixeltherefore corresponds to a pencil of parallel light. The angle whichthis pencil of light forms with the z axis of the assembly is a functionsolely of the position of the pixel along the x axis.

Birefringent lamina 10, optically uniaxial with the x axis, is cut insuch a manner that the optical axis makes an angle 0 with the x axis ofthe assembly, as is shown in FIG. 4, and the optimum is at θ=450.

In FIG. 4 the x and z axes represent the assembly axes as they appear inFIG. 3. Moreover, x′ is the direction of the optical axis and curve 12represents the ellipsoid of the indices.

Thus, it is possible to show that birefringent lamina 10 introduces anoptical delay between the two polarization components, that is, along xand y defined in FIG. 3, which delay is primarily proportional to angleβ, therefore proportional to position Xo of the pixel considered.Therefore a carrier is obtained in the plane of camera 9 whose fringesare parallel to the y axis. The increment size of the carrier is afunction of the characteristics of the lamina, of its thickness, of thebirefringence and of its cut.

The invention was described above by way of example. It is understoodthat an expert in the art is capable of realizing different variants ofthe invention without departing from the scope of the patent.

1-10. (canceled)
 11. A process for generating a carrier in aninterferogram in either of a “real-time holographic interferometry”device or a “double-exposure holographic interferometry” device,comprising: recording a first wave front on a hologram and coming froman object; generating a second wave front, which two wave fronts aresuperposed; orthogonally polarizing the superposed wave fronts, eithernaturally or by a modification stage; and passing the orthogonallypolarized two wave fronts through a birefringent crystal, a polarizerand a detector to make the two wave fronts interfere.
 12. The processaccording to claim 11, wherein the first way front is restored in thecase of “real-time” interferometry by readback of the hologram by areference beam, which first wave front is superposed on the second wavefront coming from the object observed.
 13. The process according toclaim 11, wherein the second wave front is obtained in the case of'double-exposure” interferometry by recording a new wave front comingfrom the object.
 14. The process according to claim 11, whereinmodifying comprises modification of the wave coming from the object orof the wave of the reference beam in the case of interferometry “in realtime,” or of one of the two reference beams in the case of“double-exposure” interferometry with an electro-optical switch, whichmodification of the polarization of one of the waves takes place afterrecording the hologram.
 15. The process according to claim 11, whereinholographic material used in the interferometry device is an anisotropicmaterial.
 16. A device for generating a carrier in an interferogram ofthe “real-time holographic interferometry” type or of the“double-exposure holographic interferometry” type, comprising a recorderthat records a first wave front coming from an object on a hologram, asecond wave front generator, in which two orthogonally polarized wavefronts are superposed, a plurality of lenses, at least one polarizerthrough which the two wave fronts pass, at least one holographicenvironment, at least one detector and a birefringent crystal throughwhich the two wave fronts pass.
 17. The device according to claim 16,wherein the birefringent crystal is in the form of a birefringentbiprism.
 18. The device according to claim 16, wherein the birefringentcrystal is in the form of a birefringent lamina.
 19. The deviceaccording to claim 16, wherein the birefringenet lamina is situated atinfinity, from an optical viewpoint, relative to the object observed.20. The device according to claim 19, wherein the biprism is placed inproximity of an image of the object.